docs: def get_graph_by_ids(self, network_ids: List[int]) -> BELGraph:
          """Get a combine BEL Graph from a list of network identifiers."""
          if len(network_ids) == 1:
              return self.get_graph_by_id(network_ids[0])
  
          log.debug('getting graph by identifiers: %s', network_ids)
          graphs = self.get_graphs_by_ids(network_ids)
  
          log.debug('getting union of graphs: %s', network_ids)
          rv = union(graphs)
  
          return rv